1. Field of the Invention
The present invention relates generally to an optical repeater applicable to a direct optical amplification submarine interconnection system, and more particularly to an inner unit structure in an optical repeater provided with a light amplifying medium such as an erbium doped fiber.
2. Description of the Related Art
An optical submarine interconnection system as a typical system provided with an optical repeater is one for transmitting an optical signal via a submarine cable including an optical fiber laid along a sea floor between continents or the like and for relaying the attenuated optical signal with an optical repeater set on the sea floor to thereby compensate for the attenuation or the like. In recent years, an optical repeater provided with an optical amplifier has reached a level of practical application, and the optimization of an inner unit structure in the optical repeater is being groped for.
Conventionally, a 3R type regenerative repeater is known as an optical repeater to be applied to a system including an optical fiber cable and a power cable for series constant current feed laid between a first terminal station and a second terminal station. In this optical repeater, an optical signal from an input optical fiber cable is once converted into an electrical signal by a photodetector. After the electrical signal is subjected to given processing such as waveform shaping, the electrical signal is converted again into an optical signal, which is in turn sent to an output optical fiber cable.
Further, an optical amplifier provided with a doped fiber doped with a rare earth element such as Er (erbium) as a light amplifying medium has reached a level of practical application in recent years, and an optical repeater (optical amplifying repeater) having this kind of optical amplifier is being developed.
The optical amplifying repeater has a feature, in a word, such that it has an electrical circuit greatly simplified and an optical circuit greatly complicated in comparison with the 3R-repeater. Accordingly, the optical amplifying repeater has various technical problems to be solved. The problems will now be described more specifically.
(1) The 3R-repeater employs only input and output optical fibers. To the contrary, a principal signal system in the optical amplifying repeater includes an optical circuit composed of some optical parts, so that a plurality of optical fibers for connecting these optical parts are necessary. These optical fibers are connected by splicing, for example, and the minimum bending radius of each optical fiber is defined. Accordingly, it is difficult to reduce the size of the repeater.
(2) In the optical amplifying repeater unlike the 3R-repeater, the optical circuit sometimes differs according to a short-distance transmission type or a long-distance transmission type.
(3) The interconnection interval between the optical amplifying repeaters is shorter than that between the 3R-repeaters. Accordingly, many optical amplifying repeaters must be put on a cable laying ship at a time.
(4) Since the interconnection interval is short, many optical amplifying repeaters must be manufactured.
(5) Since the interconnection interval is short, the repeaters are laid on a shallow sea floor in many cases. As a result, the temperature surrounding the repeaters is high, so that good heat radiation is required for the repeaters.